Current processes used for microcomponent fabrication, primarily based on silicon processing technology, are relatively expensive and often do not meet the production rates required for certain specific components, such as for optical component fabrication (e.g., fiber connectors and beam splitters). Furthermore, silicone-based components generally lack the desired high-temperature performance for chemical industry applications (e.g., microchannel devices for gas separation/reforming). Conventional micromachining techniques to form high-precision components (e.g., wet and dry etching) are very slow and expensive processes and often do not provide the desired production rates and cost criteria required for bulk production of components. Further, most of these processes are specific for silicon, which has a relatively low fracture toughness (0.7-0.8 MPa.m1/2) and is subject to severe corrosion in high-temperature oxidizing conditions typical of industrial chemical processes employing microchannel devices.
Optic fiber connectors based on V-groove designs are currently fabricated by etching silicon wafers. For efficient operation, the fiber axes must be precisely aligned with respect to each other to submicron tolerances. The V-groove design allows such precise alignment. Silicon wafers are usually used for this application as the end users, i.e., the optic fiber communications industry, have strong historical, commercial and technological ties with the semiconductor manufacturing industry. Silicon wafer fabrication technology is a relatively mature technology, and it can be used to fabricate microcomponents with extremely high dimensional tolerances, frequently in the nanometer range, through precisely controlled etching of single crystal silicon along crystallographic planes.
Fabrication of silicon wafers with relatively complex geometries, however, usually involves multiple steps including photolithography, wet/dry etching, anisotropic etching and reactive-ion etching. In many cases, the required production rates and market demand is greater than that achievable with etched silicon wafers. The process is further complicated if the components are designed as beam-splitters or source-to-fiber connectors where grooves in non-crystallographic directions are frequently required.
Further, there are limitations in thickness of commercially available wafers due to the difficulty of fabricating thick single-crystalline wafers, and therefore the strengths of the etched wafers themselves are not sufficient for most applications. This limitation is currently overcome by joining/laminating the wafer with secondary support materials, like polycrystalline silicon or silica, for mechanical reliability. This requirement for a mechanical support adds an additional joining step to the process.
Ceramic materials have excellent corrosion and mechanical properties that make them very attractive for high-temperature applications. However, current processing techniques for microdevices made of ceramics are even more expensive than silicon technology. Most of these processes require sintering at a high temperature accompanied by a linear shrinkage of more than 20%, a process that is very difficult to accurately model, and therefore the component dimensions are difficult to control to required tolerances.
Ceramic nanoporous materials are of tremendous interest in the chemical and transportation industry as they are ideal support materials for catalysts and adsorbents in gas-phase processes including chemical reactions, gas refinement/purification and pollution control. The current use of nanomaterials in large-scale industrial processes is primarily in the form of wet or dry packed beds. Packed beds typically consist of catalytic materials (e.g., CeO2, V2O5, Pt) doped into, or deposited on, high surface area powders of a support material like α-Al2O3. Packed bed reactors typically have very high capital and operating costs and are plagued by problems of high pressure drop and low flow rates for designs where the flow is through the bed, and by problems of low efficiency when the flow is over the bed. Further, for packed bed scrubbers used for pollution control to remove toxic gases like Nitrogen oxides (NOx) and sulfur-dioxide (SO2) from stack gases, the gases need to be cooled prior to passing through the packed bed. In automotive applications, currently, the most advanced catalytic converters use palladium and platinum on nanoporous ceramic coatings (CeO2 doped Al2O3) supported on metallic honeycomb structures. The use of the honeycomb support structures is primarily due to the technical difficulties and processing costs associated with monolithic nanoporous ceramic structures. Most conventional ceramic processing techniques to form components with stable structures involve sintering at temperatures above 1,000° C. Exposure to these temperatures significantly lowers the surface area of the fired component.
One known manufacturing technique for certain ceramic components uses phosphate bonding through a chemical reaction between a ceramic powder and phosphoric acid. Phosphate-bonded cements have been around for more than two decades and have largely been used for making refractory bricks for metallurgical furnaces. Phosphate bonded alumina ceramics have also been used for dental applications. The known ceramic component manufacturing processes have been used primarily for large-scale components, such as bricks and other large components. Limitations inherent in known manufacturing techniques render them incapable of forming components having certain desired small feature sizes being reproducible to micrometer accuracy in a molded or cast part.
It would be an improvement in the fabrication art to provide a low-temperature, net-shape (or near net-shape) fabrication technique, based on chemical reaction bonding, that is efficient, cost-effective, scalable, and more environmentally friendly. Desirably, the improved fabricating technique would produce ceramic-based components having reproducible features smaller than about one-tenth of a millimeter, and as small as one micrometer (micron, or 10−6 m), or even smaller. A desirable fabricating technique would replicate features of a mold surface with at least a one micrometer tolerance.